Process for producing 2,3,5-trichloropyridine

ABSTRACT

Production of 2,3,5-trichloropyridine by reacting 3,5-dichloro-2-pyridone at 30° to 150° with phosgene in the presence of an N,N-disubstituted formamide of the formula ##STR1## in which R 1  and R 2  can be identical or different and are each an alkyl group having 1 to 4 carbon atoms, or R 1  and R 2  together with the nitrogen atom form the pyrrolidino, piperidino or morpholino, and in the presence of an inert solvent. 
     2,3,5-trichloropyridine is a valuable intermediate for producing herbicidally active α-[4-(3&#39;,5&#39;-dichloropyrid-2-yloxy)-phenoxy]-alkanecarboxylic acids and derivatives thereof.

The present invention relates to a process for producing2,3,5-trichloropyridine.

2,3,5-Trichloropyridine is a valuable intermediate for producingherbicidally activeα-[4-(3',5'-dichloropyrid-2-yloxy)-phenoxy]-alkanecarboxylic acids andderivatives thereof. The production and use of suchα-[4-(3',5'-dichloropyrid-2-yloxy)-phenoxy]-alkanecarboxylic acids andderivatives thereof are described for example in the U.S. Pat. No.4,092,151.

The production of 2,3,5-trichloropyridine by reaction ofN-methyl-3,5-dichloro-2-pyridone with phosgene as solvent is known. Theprocess comprises introducing N-methyl-3,5-dichloro-2-pyridone into atoluene solution of phosgene, and heating the mixture in a closed systemfor 3 hours at 150° C. 2,3,5-Trichloropyridine is obtained in thismanner in a yield of about 90% of theory (Ann. Chem. 486, 78-79 (1931)).The N-methyl-3,5-dichloro-2-pyridone required as starting material forcarrying out this process is obtainable by reaction of 2-aminopyridinewith potassium nitrite in a sulfuric acid solution to 2-pyridone (Arch.Pharm. 240, 250 (1903)), chlorination of this to give3,5-dichloro-2-pyridine (J. Org. Chem. 23, 1614 (1958)), and subsequentmethylation thereof (Ann. Chem. 486, 74 (1931).

The known process is disadvantageous in so far as the3,5-dichloro-2-pyridone obtained by chlorination of 2-pyridone hasfirstly to be converted into N-methyl-3,5-dichloro-2-pyridone, and theN-methyl group of this is removed again in the subsequent reaction withphosgene to give 2,3,5-trichloropyridine. In this process, there is thusintermediately introduced a group which is not present at all in thefinal product. This is uneconomical with respect to the necessity ofhaving to carry out an additional process step, with respect to thecosts associated with that, with respect to the losses in yieldresulting from this additional step, and finally with respect to theconsumption of chemicals which do not contribute towards the structuralsynthesis of the end product. Furthermore, the high reaction temperatureat which the process is performed necessitates carrying out the reactionin a closed system under pressure, and as a result of this there is aconsiderable increase in the expenditure on equipment required to carryout the process on a large commercial scale.

The aim of the present invention therefore is to avoid the disadvantagesof the known procedure, and to provide a process by which2,3,5-trichloropyridine can be produced on a commercial scale in asimple and economic manner.

The proposal according to the present invention is to produce2,3,5-trichloropyridine by reacting 3,5-dichloro-2-pyridone at 30°-150°C. with phosgene in the presence of an N,N-disubstituted formamide ofthe formula ##STR2## in which R₁ and R₂ can be identical or differentand are each an alkyl group having 1-4 carbon atoms, or R₁ and R₂together with the nitrogen atom form the pyrrolidino, piperidino ormorpholino group, and in the presence of an inert solvent.

The reaction of 3,5-dichloro-2-pyridone is performed within theabove-given temperature range of 30°-150° C., preferably at 50°-90° C.The reaction is performed as a rule under normal pressure. It may benecessary to carry out the reaction in a closed system under pressureonly when a relatively low-boiling solvent is used and a high reactiontemperature applied, for example a temperature in the range of 100°-150°C.

Suitable solvents in which the process according to the invention can beperformed are in general those solvents which under the reactionconditions are inert to the reactants. Suitable solvents are inparticular: aromatic hydrocarbons, such as benzene, toluene or xylene;aliphatic hydrocarbons, such as hexane, heptane or petroleum ether;cycloaliphatic hydrocarbons, such as cyclopentane and cyclohexane;halogenated aromatic hydrocarbons, such as chlorobenzene oro-dichlorobenzene; halogenated aliphatic hydrocarbons, such as methylenechloride, chloroform, carbon tetrachloride, 1,2-dichloroethane andtrichloroethylene, as well as aliphatic carboxylic acid esters, such asethyl acetate and isopropyl acetate. A preferred solvent is toluene.

Suitable N,N-disubstituted formamides are N,N-dimethylformamide,N,N-diethylformamide, N,N-dipropylformamide, N,N-dibutylformamide,N-methyl-N-butylformamide, as well as N-formylpyrrolidine,N-formylpiperidine and N-formylmorpholine. A preferred N,N-disubstitutedformamide is N,N-dimethylformamide. The N,N-disubstituted formamides areused in amounts of 0.01-1.0 mol per mol of 3,5-dichloro-2-pyridone,preferably 0.05 to 0.15 mol per mol of 3,5-dichloro-2-pyridone.

Phosgene is used according to the invention generally in at least anequimolar amount or in excess. The amount advantageously used is 1.0 to1.5 mols of phosgene per mol of 3,5-dichloro-2-pyridone. The amountpreferably used is 1.0 to 1.3 mols of phosgene per mol of3,5-dichloro-2-pyridone. After completion of the reaction, the unreactedphosgene is reacted with an aqueous caustic solution, such as with anaqueous sodium hydroxide solution or with aqueous ammonia.

The reaction of 3,5-dichloro-2-pyridone with phosgene takes as a rule1-5 hours, and in most cases it is completed in 2-4 hours.

According to a preferred embodiment of the process of the invention,3,5-dichloro-2-pyridone is suspended in toluene, the water present isseparated by azeotropic distillation, 0.05-0.15 mol ofN,N-dimethylformamide per mol of 3,5-dichloro-2-pyridone is added, and1.0-1.3 mols of phosgene per mol of 3,5-dichloro-2-pyridone areintroduced at 75°-80° C. After the addition of phosgene has beencompleted, the reaction mixture is allowed to react for a further onehour at 75°-80° C., and the excess phosgene is subsequently reacted withaqueous ammonia. The phases are afterwards separated, the solvent isdistilled off from the organic phase, and 2,3,5-trichloropyridine isobtained as a melt.

It becomes possible by the process according to the invention to produce2,3,5-trichloropyridine on a commercial scale in a simple and economicmanner. The process avoids the N-methylation of 3,5-dichloro-2-pyridonewhich the known process necessitates, and can be performed underconsiderably milder conditions. The total yield is increased, and thenecessary expenditure on equipment to carry out the process is reduced.

The process according to the invention is further illustrated by thefollowing Example.

EXAMPLE

82.0 kg of 3,5-dichloro-2-pyridone, 330 kg of toluene and 3.8 kg ofN,N-dimethylformamide are placed into a 500 liter enamelled vessel. Themixture is then heated with stirring to 75°-80° C., and at thistemperature 60.0 kg of phosgene are introduced in the course of 2-3hours. After completion of the addition of phosgene, stirring iscontinued at 75°-80° C. for 1 hour. The reaction mixture is afterwardscooled to 20°-25° C., and is subsequently stirred up with 12.0 kg of 25%aqueous ammonia. After separation of the aqueous phase, the toluene isdistilled off at normal pressure. The yield as residue is 92.0 kg of2,3,5-trichloropyridine (95%) [96% of theory] in the form of a melt,which solidifies on cooling. The product melts at 47°-48° C.

What is claimed is:
 1. A process for producing 2,3,5-trichloro-pyridinewhich comprises suspending in a mixture of toluene and water,3,5-dichloro-pyridone, the water present is separated by azeotropicdistillation, adding 0.05-0.15 mol of N,N-dimethylformamide per mol of3,5-dichloro-2-pyridone introducing, 1.0-1.3 mols of phosgene per mol of3,5-dichloro-2-pyridone at 75°-80° C., stirring the reaction mixturefurther for 1 hour, reacting the excess phosgene with aqueous ammonia,separating the phases, and obtaining from the organic phase, afterremoval of the solvent by distillation, the formed2,3,5-trichloro-pyridine in the form of a melt.